Digital cameras include a two-dimensional photo-detector array. A single pixel can include one or more photo-detectors as well as multiple transistors. Typical photo-detectors include photodiodes, phototransistors, photo-gates, hole accumulation diodes, pinned diodes, avalanche diodes, buried accumulation and transfer layer devices.
Readout circuits read the analog signal (current or voltage) outputted from the pixel. Typically, each column of pixels is connected to a column readout circuit. The column readout circuit includes an analog to digital converter or is connected to an analog to digital converters.
Many vendors utilize sigma delta analog to digital converters. FIG. 1 illustrates a prior art analog to digital converter 10. Analog to digital converter 10 includes an input node 12, integrating capacitor Ci 14, quantizer 18, switch 16 and reference current source 30. Switch 16 is controlled by quantizer 18 and is connected between reference current source 30 and integrating capacitor Ci 14. Integrating capacitor Ci 14 is also connected to input node 12 and to the input of quantizer 18. Quantizer 18 samples the voltage on integrating capacitor Ci 14 and closes switch 16 (thus connecting reference current source 30 and integrating capacitor Ci 14) when the sampled voltage is below a threshold. A control signal sent by quantizer 18 to switch 16 is the output signal of analog to digital converter.
Integrating capacitor Ci 14 is drained by input analog current Iin 22 and selectively charged by reference current Iref 32. Iin 22 represents the analog signal of a pixel that should be read by a column readout circuit.
Quantizer 18 samples the voltage on integrating capacitor Ci 14 in response to a reception of a jittered clock signal 150.
The reference current source 30 provides an ideally stable reference current to integrating capacitor Ci 14. The charge provided to the integrating capacitor Ci 14 depends upon the value of Iref 32 and the integration period. The integration period substantially equals the period of jittered clock signal 150.
Large photo-detector arrays typically have more than one thousand columns, and accordingly are connected to more than a thousand analog to digital converters. Due to clock signal jitter different analog to digital converters have different integrating periods and accordingly provide non-uniform readout results.
There is a growing need to provide analog to digital converters that are more tolerant to clock signal jitters.
Analog to digital converters are subjected to various noises and phenomena that can affect their performance. Analog to digital converters and especially those which are included in devices that include multiple analog to digital converters are susceptible to IR drops. The IR drops effect analog to digital converters that include (or are connected to) continuous current sources. IR drops can be responsive to current drained by different analog to digital converters that operate concurrently. Gain mismatches and cross-talks can occur in devices that include multiple analog to digital converters.
There is a growing need to provide a more robust analog to digital converters.